1. Field of the Invention
The present invention relates generally to the field of semiconductor manufacture. More particularly, the present invention relates to a method for forming trenches and trench isolation on a semiconductor substrate.
2. Description of the Prior Art
As known in the art, shallow trench isolation (STI) structures are usually used in combination with deep trench isolation (DTI) structures in the manufacture of semiconductor devices in order to electrically isolate adjacent devices from each other, for example, for the isolation between adjacent image sensor pixels.
FIGS. 1-9 illustrate a conventional method for forming trenches and trench isolation on a substrate. In FIG. 1, a pad oxide layer 22 and a pad nitride layer 24 are formed on the substrate 12. Using lithographic and etching processes, shallow trenches 26 are formed in the pad nitride layer 24, the pad oxide layer 22 and the substrate 12 within the first area 101, for example, the logic circuit area of an image sensor device. An oxide liner 26a is then formed on the surface of each of the shallow trenches 26. In FIG. 2, an oxide layer 30 is deposited to fill the shallow trenches 26 and is then densified. In FIG. 3, a reverse mask is typically used for removing a portion of the oxide layer 30 within the second area 102, for example, a pixel array area of an image sensor device. In FIG. 4, a chemical mechanical polishing (CMP) is carried out to remove the oxide layer 30 outside the shallow trenches 26 to thereby forming the shallow trench isolation (STI) structures 28. In FIG. 5, a hard mask 40 is deposited to cover the pad nitride layer 24 and the STI structures 28. In FIG. 6, using lithographic and etching processes, openings 40a are formed in the hard mask 40, the pad nitride layer 24 and the pad oxide layer 22 within the second area 102. In FIG. 7, through the openings 40a, the substrate 12 is etched to form deep trenches 46. In FIG. 8, an oxide liner 46a is formed on the surface of each of the deep trenches 46. An oxide layer 50 is deposited to fill the deep trenches 46, and is then densified. In FIG. 9, a CMP process is carried out to remove the oxide layer 50 and the hard mask 40 outside the deep trenches 46, thereby forming deep trench isolation (DTI) structures 48.
The above-described conventional method for forming trenches and trench isolation has several drawbacks. First, the conventional method is complicated and time-consuming because the shallow trenches 26 and the deep trenches 46 are formed separately in different stages. Two etching steps and two liner oxidation steps are required to complete the shallow trenches 26 and the deep trenches 46. In addition, it requires two gap-filling steps and two densification steps to complete the STI structures and the DTI structures. Second, a reverse mask is required to reveal the oxide layer 30 within the second area 102 and a subsequent dry etching process is needed to etch away a portion of the exposed oxide layer 30 from the second area 102 in order to avoid loading effect during CMP process or the risk of incomplete removal of the oxide layer 30 on the pad nitride layer 24. The reverse mask adds to the cost of the semiconductor manufacture.